feature: rsync

[martindurant]

Idea: to make a method which works something like a low-featured version of rsync, to make local and remote sets of files equivalent (normal filesystem) or between filesystems (generic filesystem).

This would work the same as get/put(recursive), but skip files which are already in place. A target file will be deleted/replaced if (options):

• the source file is missing ("cull")
• the source file has a different size ("different") - default
• the source file is newer ("latest")
• "never"
• perhaps some form of checksum, if the remote system supplies it.

[ianthomas23]

rsync was added in PR #1147.